Human and mouse cDNAs have been studied in projects aimed at elucidating human and mouse genomes. Mouse genomes are very similar to human genomes; genes isolated in mouse can be used directly in studies on human diseases; and mice are generally used as research models for study of human diseases. In particular, studies are comprehensive analytical methods of analyzing the difference in gene expression in different cells by using a mouse cDNA microarray, and the difference in gene expression in organs related various human cancers, i.e. between cancerous tissues and normal tissues. As a result, various new cancer-related genes have been identified. For example, various tumor suppressor genes were identified and analyzed, and proved to be effective in suppressing cancer and thus, useful as a therapeutic target genes (genes such as P53, P16waf-1, Rb, PTEN, TSLC-1, and KAI-1) in preclinical tests (animal tests).
Sequences of a great number of various genes have been determined by comprehensive analysis of the gene sequence of full-length cDNA. However, the functions of most of the genes in the body remain unidentified even now (see, for example, Okazaki, Y., Hayashizaki, Y. et al.: Analysis of the mouse transcriptome based on functional annotation of 60,770 full-length cDNAs Nature 420(6915), 563-573 (2002), Strausberg, R. L. et al.; Generation and initial analysis of more than 15,000 full-length human and mouse cDNA sequences Proc. Natl. Acad. Sci. U.S.A. 99(26), 16899-16903 (2002)).
Gene therapy is currently practiced as one of cancer therapies. However, it is only possible to apply a viral vector only in the region of target cell by the gene therapy, because gene introduction is performed by using a nonproliferating viral vector for safety, and thus, the incorporation efficiency is low, making it difficult to obtain a satisfactory treatment effect.
It is also known that telomerase activity is often raised in cancerated or immortalized cell lines, while the telomerase activity is almost undetectable in normal somatic cells other than germ cells, blood cells, epithelial stem cells and the like.